Aerith's is more symbolic and shocking.
But Zacks? You play the entire game knowing he is going to die, that Aerith will move on, then she will die never knowing what really happened to him, that his parents will never know where he is and wait for him, and that the people he works for will betray him. It's so different I don't know how to compare it to Aerith's.
There's the matter of actually playing him as he is dying with the game mechanics breaking down as you fight. That part was sadder than the actual ending, watching the DMW go on the fritz.
But I think the only thing that really sours it is the last minute smile, hand, happy music thing. That's a mood breaker.
Anyway, I'd vote for Zack but there's a part of me that still wants me to vote for Aerith. For everything it encompasses in the story and the characters.